UV Mapping to meshes
Whatever material you currently have on your mesh is the default color that object would be.  Something like this:



Go into Edit Mode, and Unwrap your object (I used Smart UV Unwrap):



Open up your image in the UV Image editor, and align your UVs to where you would like.  Little inconsistency I accidentally put in, move the little group of squares outside of the image, this is required for the clipping:



Now go to the texture tab, and set the mapping to UV (from Generated) and select the UV Map you want to use for that image (don't forget to open that image in the texture tab).  I set it to Clip instead of Repeat for the texture mapping:



I set the diffuse color of the material (back in that tab) to be red, so now the scene has a cube with the texture used on one side of it, but the color red is used on the other parts, resulting in something like this (in the rendered view):



All of this combined shows you how to add a texture to a model already built, and maintaining the material for the other portions.  It will be affected by the specularity of the material if you have adjusted that.